In the field of orthopedics, spinal rods are often implanted on at least a temporary basis to hold and support spines for various medical reasons. For example, curvature of the spine may be corrected by means of a rod, and severe fractures may be dealt with by providing support through one or more rods.
These rods are connected to the spine by means of known spinal hooks or spinal screws, for example, open screws or closed screws. Open screws define a head and a separable cap or cover to capture the rod between them onto the screws. The head itself defines a U-shape which terminates in lipped edges, being attached to the screw. Channels on the cap slide laterally onto and along the lipped edges of the U-shaped head of the screw to capture the spinal rod between them. Spinal hooks may carry heads of similar design, and are included by implication in the further discussion of the prior art.
Disadvantages exist in this construction. First, the spinal screws must be placed into various vertebra of the spine, and then the rod is placed into recesses defined by the U-shaped heads on the ends of the screws. However, typically, the spinal rods must be deeply seated into the U-shaped heads, or the cap can be applied with the lateral sliding application only with great difficulty. If the rod is not completely seated, but is spaced from the bottom of the U-recess of the spinal screw, the surgeon may find it nearly impossible to apply the cap by the lateral sliding action previously described.
Also, in the prior art spinal screws, a set screw is provided at the center of the cap. This set screw initially projects upwardly, and then is screwed to move downwardly into frictionally retentive relation with the spinal rod within the U-shaped head and the cap. It is desirable for the length of the spinal screws to be at an absolute minimum so they fit into the surgical incision without the need to enlarge the incision. The positioning of the set screw in the prior art undesirably increases the vertical profile of the spinal screws.
Also, in the conventional, open spinal screws described above, lateral retention force is provided to the rod by the set screw, forcing the rod deeply into the engagement with the walls of the U-shaped recess of the screw head. However, a substantial portion of the periphery of the rod within the screw head is not held under compressive contact by either the set screw or the recess walls. Thus, the possibility of frictional slippage of the spinal rod is substantial with respect to conventional spinal screws.
Another type of spinal screw is the closed screw, where the screw head carries a preformed aperture without a removable cap. A set screw, coaxial with the bone screw, then presses the rod within the aperture of the screw head against an opposed aperture wall for retention of the rod.
Closed screws exhibit some disadvantages similar to the open screws. Also, it is of course impossible to apply a spinal rod laterally into engagement with a closed spinal screw. The end of the spinal rod must be threaded through the preformed aperture, and, in many surgical situations, that is not possible.
The prior art also utilizes links for connecting surgical rods to bone in which a hook rather than a screw is used. Apart from that, a head is provided with an aperture which may be a preformed aperture, or may have a cap of the type described above.
Another type of bone screw is disclosed in Vignaud et al. U.S. Pat. No. 5,176,680.
In accordance with this invention, a link is provided for connecting surgical rods to bone, in which the above disadvantages are effectively eliminated, providing substantially improved ease of installation during the surgical process, coupled with improved retentive characteristics shown by the links of this invention for connecting the surgical rods to bone.